By way of definition, the term “tachyarrhythmia” refers to fast, abnormal rhythms of a heart chamber that include atrial flutter or fibrillation (AF) and ventricular flutter or fibrillation (VF), which may be amenable of conversion to a normal sinus rhythm (NSR) by “cardioversion” or “defibrillation”, and tachycardias that may be amenable to conversion to NSR by the application of certain ATP regimens to the affected heart chamber. Such tachycardias include supra-ventricular tachycardia (SVT) originating from one or more ectopic site in the atria, and ventricular tachycardia (VT) originating from one or more ectopic site in the ventricles. Individuals whose heart's go from sinus rhythm into high rate, non-sinus rhythm described as atrial tachycardia(AT—a form of SVT) and VT can suffer debilitating physiologic effects that in certain cases can progress to more dangerous high rate VT or VF that lead to sudden cardiac death (SCD) unless the ventricles are cardioverted or defibrillated within a very short time after onset of such high rate VT or VF.
High rate, non-sinus, AT and VT episodes can often be terminated by the application of respective atrial and ventricular ATP regimens that comprise of train of pacing pulses delivered at a rate faster than the AT or VT rate. Each such train is referred to as an ATP sequence. A number of such ATP sequences in succession comprise a single ATP therapy and can be used to treat an AT or VT. However, efficacy of ATP regimens is not as high as the efficacy of alternative shock therapy, and hence there is always a chance that ATP may not be effective. To address this risk, automatic implantable arrhythmia control devices, particularly implantable cardioverter/defibrillators (ICDs) that provide staged or tiered therapies always include cardioversion/defibrillation regimens following an ATP. Among the most important functions of ICDs are to sense cardiac signals comprising the P-wave and/or R-wave, to detect and correctly identify the particular tachyarrhythmia that is occurring as a function of meeting specific detection criteria, to supply an appropriate cardioversion/defibrillation or ATP burst pacing regimen, and to determine whether or not the supplied regimen was effective. Current ICDs employ tachyarrhythmia classification algorithms that generally characterize heart rates between 150 and 250 bpm as tachycardias that can be further differentiated by their EGM morphology as either monomorphic or polymorphic. Atrial or ventricular arrhythmias exhibiting heart rates above the upper AT or VT range are typically classified as AF or VF, respectively.
The atrial and ventricular ATP regimens when applied to the atria and/or the ventricles to counter and convert AT or VT episodes to normal sinus rates can be highly efficacious and provide the advantage of being painless in contrast to the shock therapy, which while being even more efficacious can be quite painful. Parameters of the ATP regimens, including the pacing pulse amplitude and pulse width, the number of pacing pulses delivered in a given ATP, the inter-pulse interval between the pacing pulses and the coupling interval between the detected atrial P-wave or ventricular R-wave and the first delivered ATP pulse are programmed by the attending physician in patient work-ups to determine effective parameter values.
Typically, a series of ATP regimens are programmed to be delivered in succession to address the scenario that initial ATP regimens may not necessarily be effective in terminating the AT or VT episode. A termination algorithm is invoked by the ICD operating system upon delivery of the last pacing pulse of an ATP burst, and the delivery of one or more progressively more “aggressive” ATP burst regimens e.g., having an increased number of pace pulses and/or reduced inter-pulse interval, can be delivered depending upon the pre-programmed sequence. Typically, termination is confirmed by a return to NSR defined as a sequence of a predetermined number of spontaneous depolarizations (P-waves or R-waves) separated by greater than a defined interval.
Such an ICD providing burst ATP regimens is disclosed in commonly assigned U.S. Pat. No. 6,167,308 that is directed to an ICD or implantable anti-tachycardia pacemaker that modifies the ATP regimen for subsequent deliveries based on history, and which when continued in its original configuration would be unlikely to terminate the detected tachycardia.
As set forth in the article, “Characterization of return cycle responses predictive of successful pacing-mediated termination of ventricular tachycardia” by Callans et al, published in the Journal of the American College of Cardiology, Vol. 25, No. 1, January, 1995, pp 47-53, the pattern of return cycle length (the time period between the last pulse of an ATP and the first spontaneous depolarization detected post-ATP) for a series of ATP sequences can be predictive of the likelihood of success of a longer series of overdrive pacing pulses having the same parameters.
In the above-referenced '308 patent an ICD switches from a first programmed ATP regimen to a second programmed ATP regimen if the return cycle length (RCL) indicate that the ATP regimen being delivered is unlikely to be successful in terminating the tachycardia. In order to accomplish this result, an initial short series of pacing pulses of an ATP regimen are delivered, for example two to four pacing pulses, and delivery is then interrupted to measure the RCL. A second sequence of slightly different ATP regimen is then delivered, for example an ATP with one pulse incremented, and delivery is again interrupted to measure the RCL.
The second measured RCL would be expected to exceed the first measured RCL by a threshold amount if the burst pacing pulses were interacting with VT wavefront in a way that the conversion of AT or VT to NSR was likely. In that case, the aggressiveness (i.e. cycle length (CL) or inter-pulse interval) is maintained the same as the previously delivered ATP and a few more pulses are added to the subsequently delivered ATP sequence. As described before, a termination algorithm is invoked after ATP delivery to determine if the delivered ATP regimen was successful or unsuccessful in converting the tachycardia episode to NSR.
By contrast, it is presumed that the aggressiveness of the ATP regimen, is not sufficient to interact appropriately with the AT or VT and terminate the episode if the second measured RCL does not exceed the first measured RCL by a threshold amount, and the next programmed ATP therapy is typically more aggressive but has the same number of pulses as the previous ATP. If the ATP aggressiveness exceeds a pre-assigned threshold, no more ATP sequences are delivered for safety reasons and a cardioversion shock is delivered to terminate the episode.
This approach advantageously attempts to deliver an optimally aggressive ATP regimen. An optimal aggressiveness for the delivered ATP presumably can result in overall higher efficacy of ATP because it is presumed that an overaggressive ATP with empirically determined number of pulses can re-induce AT or VT that has been successfully terminated by the same ATP (e.g. initial pulses can terminate the tachycardia but later pulses might start it).